In order to meet the request that semiconductor devices are made finer and caused to have higher functions, the wiring width of power supply lines arranged in the entire area of the main faces of their semiconductor chips (semiconductor elements) or the interval between signal lines arranged therein has been becoming narrower. For this reason, the impedance thereof increases or signals between signal lines of different nodes interfere with each other so as to cause hindrance to the exhibition of sufficient performances for the operation speed of the semiconductor chips, the margin of the operating voltage thereof, the resistance thereof against damage by electrostatic discharge, and others. In order to solve these problems, package structures wherein semiconductor elements are laminated are suggested (see JP-A-55-111151 and JP-A-2002-261233).
As a material used to stick semiconductor elements to a substrate or the like, the following examples are suggested: an example wherein a thermosetting paste resin is used (see, for example, JP-A-2002-179769); and examples wherein an adhesive sheet composed of a thermoplastic resin and a thermosetting resin is used (see, for example, JP-A-2000-104040 and JP-A-2002-261233).
In conventional processes for producing a semiconductor device, an adhesive sheet or an adhesive is used to adhere semiconductor elements onto a substrate, a lead frame or semiconductor elements. The adhesion is performed by attaching the semiconductor elements to the substrate or the like under pressure (die attaching) and then curing the adhesive sheet or the like in a heating step. Furthermore, wire bonding is performed in order to connect the semiconductor elements electrically to the substrate or the like. Thereafter, a sealing resin is molded onto the resultant product, and post-cured to perform sealing based on the sealing resin.
However, when the wire bonding is performed, the semiconductor elements on the substrate or the like are shifted by ultrasonic vibration or heating. Conventionally, therefore, it is necessary to perform a heating step before the wire bonding so as to heat and cure the thermosetting paste resin or thermosetting adhesive sheet, thereby sticking/fixing the semiconductor elements so as not to be shifted.
An adhesive sheet made of a thermoplastic resin or an adhesive sheet composed of a thermosetting resin and a thermoplastic resin is required to undergo a heating step after die attaching and before wire bonding in order to ensure adhesive force thereof onto an object which is to be stuck with the sheet, or improve the wettability thereof onto the object.